1. Field of the Invention
The present invention relates generally to devices for sensing the speed of a motor, and more particularly to devices for sensing motor speed that retrofit into existing mechanical tachometer systems.
2. Description of the Prior Art
Devices for sensing the speed of a motor have long been in use and encompass a variety of approaches for detection of the speed of the motor.
A popular prior art approach for sensing the speed of a motor is the mechanical arrangement in which a flexible drive shaft rotating in proportion to the speed of the motor is coupled to a gear and pinion to a linkage mechanism carrying a plurality of flyweights. The faster the drive shaft rotates the further the flyweights move outwardly from a center point due to the centrifugal force experienced. A sensor is connected to a display needle which moves the needle in response to the outward movement of the flyweights.
The following patented prior art approaches use a slotted disc for generating a digital signal corresponding to the speed of a rotating shaft or the like: Metz, U.S. Pat. No. 3,729,996 issued on May 1, 1973; Swiden, U.S. Pat. No. 3,886,354 issued on May 27, 1975; and Ford, U.S. Pat. No. 3,886,120 issued on Feb. 11, 1975. A similar arrangement to those set forth above is the U.S. Pat. No. 3,942,112 issued to Westbrook on Mar. 2, 1976 which also discloses a novel rotary seal.
Such prior art mechanical systems, however, are not accurate and are not instantaneous in response to variations in the motor's speed. In the prior art electronic control systems, highly accurate and rapid determinations of the speed of the motor are made. Such control systems require a series of electrical pulses whose frequency is proportional to speed of the motor. An example of an electronic control system is disclosed in co-pending application Ser. No. 691,724, entitled "Motor Brake Control System."
None of the above prior art approaches, however, provide an apparatus with an electronic output that is retrofittable into a conventional mechanical sensor attached to a motor while preserving the mechanical output as set forth in the above application.